The infrared (IR) spectra of size-selected phenol-ammonia clusters, PhOH-(NH3)(n)(n <= 11) in the 3-10 mu m wavelength region were measured to investigate the critical number of solvent molecules necessary to promote the ground state proton transfer (GSPT) reaction. While the N-H stretching vibrations did not provide clear information, characteristic changes that are assigned to the GSPT reaction were observed in the skeletal vibrational region. The production of phenolate anion (PhO-), which is a product of the GSPT reaction, was established from the appearance of characteristic bands assignable to C-C stretching and C-H bending vibrations of PhO- and from the corresponding disappearance of C-O-H bending vibration of PhOH at n = 9. The mid-IR spectroscopy directly proves the structural change induced by the deprotonation from the O-H bond and thus establishes the GSPT reaction as complete at n = 9. No such absorptions were observed for n 5 in line with a previous report. For n = 6-8, both the proton transferred and the nontransferred signatures were observed in the spectra, showing coexistence of both species for the first time.